Industries such as the offshore oil and gas exploration and production industry or the marine renewable energy industry require subsea infrastructure and facilities to support the offshore operations, including for example manifolds, trees, riser arches, seabed foundations and pipelines. One example of an item of infrastructure is a subsea manifold, which provides an interface between pipelines and wells at the seabed. A manifold may be designed to handle flow of produced hydrocarbons from multiple wells and direct the flow to several production flow lines. A typical manifold will comprise flow meters, control systems and electrical and hydraulic components. The manifold supports and protects the pipelines and valve system, and also provides a support platform for remotely operated vehicle (ROV) operations. Manifolds and other items of infrastructure have a significant weight and size which introduce complications to the installation process.
Manifolds and other items of subsea infrastructure are manufactured onshore and transported to an installation site by a marine vessel. A conventional method of installation involves transportation of the load on the deck of a vessel until it is in the vicinity of the installation site. The load is then lifted from the deck of the vessel by a crane and lowered to the body of water until it is suspended. The load will then be maneuvered into its desired location by a marine vessel, before the load is landed on the seabed in its designated position.
Such an installation method has a number of drawbacks. For example, the weight and size of the load is inherently limited by the capacity and reach of the crane. In addition, where installation is required in deep water, the weight of the crane wire contributes significantly to the load on the crane, which reduces the effective crane capacity. Although the effects of crane wire weight can be eliminated by using weight neutral crane wires, these have the disadvantage that they contribute to the complexity of the operation and may add to the duration of the installation process. During the lifting process, dynamic and hydrodynamic loading on the vessel can be significant, which also requires a reduction in the effective crane capacity.
This type of installation method also exposes the apparatus being lifted to wave slamming as the load passes through the splash zone and water surface. Many items of subsea infrastructure comprise sensitive equipment which may be exposed to risk of damage from wave action. In addition, weather limitations may be imposed to avoid exposure of the load to large accelerating or decelerating forces during pick-up or landing on the seabed or deck of a vessel which may cause damage to the equipment. To address this, many cranes are provided with active heave compensation systems that will allow the soft landing of loads, but such active heave compensation systems can be deficient when used in deep water operations.
A heavy lift vessel (HLV) may be used to overcome some of the difficulties described above to install large and/or heavy payloads. However, an HLV requires multi-reeved crane blocks with slow hoisting and lowering speeds. The payloads are lowered or lifted very slowly, which increases the time during which the equipment is exposed to risk of damage at or near the water surface.
The problems described above are affected by sea state, with adverse environmental conditions further reducing the crane capacity and the time in which the marine vessel is able to work. Increasing sea state also increases the risk of damage to the load. Failure of the lifting system is potentially catastrophic to the load and may endanger the marine vessel and/or its crew.
To alleviate the drawbacks of the described installation method, suspended tow systems have been devised. In a direct suspension system, the load is lifted and lowered into the body of water and suspended directly below the transportation vessel. The suspension system is provided with means for resisting the full hydrodynamic loading associated with the vessel and wave motion. A direct suspension system has many of the limitations of the conventional surface transportation described above, but has the advantage that the in air lift and lowering through the water surface can be done near shore in sheltered waters. This reduces the dynamic loads and therefore may be performed with reduced crane capacity. In addition, the point from which the load is suspended is usually close to mid-ships, and is therefore subject to lower dynamics due to the pitch and roll of the vessel. However, the operation remains highly weather sensitive, due to the suspension of the load directly beneath the vessel throughout the transportation phase. The process also has the disadvantage that the additional inshore lift suspension operation is required.
A W-suspension method is an alternative to the conventional installation and direct suspension methods described above. A W-suspension method provides buoyancy tanks on the payload such that it is slightly positively buoyant. The load is connected fore and aft to tug vessels via tow lines, and is launched by towing the load at the surface until there is sufficient draught. Clump weights are then added to the tow wires to cause the structure to submerge below the surface. The depth of the structure below the surface is controlled by the length and tension of the tow lines. The load is then towed to the vicinity of the installation site, and the tow lines can be paid out until the clump weights come to rest on the seabed. Final landing of the load is achieved by flooding the buoyancy tanks to overcome the positive buoyancy.
The W-suspension method has the advantage that the need for a crane vessel is avoided, and the transition through the water surface may be performed near shore in sheltered water. Because the structure is towed in a submerged position, the transportation phase is less weather sensitive. In addition, hydrodynamic loading on the structure is reduced due to the coupling of the structure to the vessels via clump weight tow wires. GB 1576957 relates to a W-suspension system for submerging and raising a buoyant object by the deployment of clump weight chains from vessels. The chains are fixed to the corners of the load and are attached to jibs on vessels.
However, the W-suspension method has the disadvantage that it requires buoyancy tanks, which must be integral with the payload or temporally coupled to it. Where integral buoyancy tanks are provided, the structure becomes larger and heavier. Where temporary buoyancy tanks are provided, they will need to be recovered subsequent to the operation. The buoyancy tanks are subject to hydrostatic loading which limits the depth to which the method can be used. The lateral position of the structure during final lowering can be difficult to control via the clump weights, particularly in areas with strong currents. The position of the two tug vessels needs to be carefully controlled. Finally, in the W-suspension system, failure of the buoyancy tanks is catastrophic to the load.
WO 06/125791 discloses an installation system which uses a positively buoyant submerged installation vessel. A J-shaped catenary chain controls the buoyancy and depth of the installation vessel in a similar manner to a W-suspension system. The load is lowered to the seabed by paying out a line from a winch system in the vessel. The requirement for a winch is a disadvantage, as it adds to the weight and complexity of the vessel. The system also relies on buoyancy tanks. Failure of the winch system or buoyancy tanks is catastrophic to the operation.
US 2003/221602 discloses an alternative installation system, which is based in part on the W-suspension system described above. A clump weight chain is used to adjust the vertical position of a load which is suspended by buoyancy tanks. The load is suspended to a depth beneath the buoys which is greater than the distant between the buoy and the centre of the clump weight. This allows lowering of the clump weight to the seabed to ensure landing of the load. This system suffers from the drawback that the length between the buoyancy and the bottom of the load must exceed that of the clump weight if the load is to be landed. This also means that there is no provision for parking the system; the load must be lowered on to the seabed if the operation is to be interrupted. U.S. Pat. No. 5,190,107 discloses a similar system, which includes provision for anchoring the system to the seabed using a separate clump weight.
A further alternative system for lowering large structures on to the seabed is described in U.S. Pat. No. 4,828,430. The load is lifted from the vessel by a crane and lowered through the surface of the water. The load has an integral buoyancy tank which provides a small positive buoyancy. The load is lowered from surface and to the seabed by overcoming the buoyancy using a weight lowered from the crane on to the load. However, the arrangement of U.S. Pat. No. 4,828,430 relies on an integral buoyancy tank in the load, which adds to the size and weight. The installation method also requires a crane for the initial lift phase from the deck of the vessel to the body of the water, and is subject to the limitations of the conventional surface transport method described above.
It is one aim of the invention to provide a method and apparatus which overcomes or alleviates at least one drawback of each of the systems described above.
Additional aims and objects of the invention will become apparent from reading the following description.